1. Field of the Invention
This invention relates to switched-mode power converters and more specifically to an improved winding structure for the magnetic core that reduces eddy currents.
2. Description of the Related Art
Power converters are key components in many military and commercial systems and often govern size and performance. Power density, efficiency and reliability are key characteristics used to evaluate the characteristics of power converters. Transformers and inductors used within these power converters may be large and bulky and often limit their efficiency, power density and reliability. These deficiencies can be improved by using a high-frequency “switch-mode” architecture instead of traditional step-down configuration and by replacing conventional core-and-coil designs with “planar integrated magnetics.”
Planar integrated magnetics offer several advantages, especially for low-power dc-dc converter applications, such as low converter profile, improved power density and reliability, reduced cost due to the elimination of discrete magnetic components, and close coupling between different windings. For example, the integrated magnetics 10 shown in FIG. 1 for a current-doubler rectifier (CDR) comprises an E-core 12 and plate 14 wound with split-primary windings 16 and 18, secondary windings 20 and 22, and an inductor winding 24 (See U.S. Pat. No. 6,549,436). This type of core arrangement is referred to as an E-I core. Other core geometries, for example circular core legs, are also possible. The windings perform the functions of both the transformer secondary and the two inductors used in the CDR. The center-leg winding is used to increase the effective filtering inductance and carries the full load current all the time. The center leg is typically gapped to prevent core saturation.
As shown in FIGS. 2a and 2b, integrated magnetics 10 is implemented with a multi-layer printed circuit (PCB) 26 having copper traces that form the various “horizontal” windings in the plane of the PCB. Horizontal windings refer to the configuration in which the winding is oriented parallel to the core plate. In one embodiment, E-core 12 is positioned underneath the PCB so that its outer legs 28 and 30 extend through holes in the PCB that coincide with the centers of primary and secondary windings 16 and 20 and 18 and 22, respectively, and its center leg 31 extends through a hole that coincides with inductor winding 24. Plate 14 rests on the outer legs forming an air gap 32 with the center leg. In another embodiment, the E-I core is attached to the circuit board, and attaching the winding terminations to the circuit board traces completes the circuit.
The converter efficiency depends on a number of factors, including the DC and AC resistance of the windings. The DC resistance is essentially determined by the cross section of the winding. To minimize the DC resistance, the windings typically almost fill the core window, having a minimal spacing of about 10 mils from the center and outer legs. The AC resistance is a function of skin depth at a given frequency and magnetic field impinging on the windings, which leads to non-uniform currents and eddy currents that may circulate in the windings. The impedance of the winding in the layered stack determines the current it carries. To minimize the AC resistance, the core and winding structure should be designed to avoid eddy currents, the fringing magnetic field on the winding should be minimized, and the winding impedances should be balanced to ensure equal current sharing.
However, as layers are added to provide the additional windings, which are required at higher output currents to reduce net dc winding resistance, to increase the core window area utilization or to interleave primary and secondary windings, the thickness of the multi-layer PCB increases. Some of the windings layers will inevitably be close to the air gap 32 where the strong fringing flux surrounding the air gap will induce eddy currents in the windings and also lead to non-uniform winding impedance. This in turn increases total winding losses and lowers converter efficiency.
As shown in FIG. 3, a typical distribution of the magnetic field 34 within one side of the window area of the magnetic core is very strong around the air gap 32. In this particular design, the fringing field extends vertically to about one-third of the window height from the air gap and horizontally to about one-fourth of the window width. As shown in FIG. 4, the field lines 36 of the magnetic field are generally perpendicular to the plane of the horizontal windings, especially near the air gap 32 and thus induce large eddy currents 38 in the entirety of the windings. These losses reduce the efficiency of the power converter. Furthermore, because the current in the windings closer to the air gap is attracted towards the high field, the currents flowing through them and consequent losses are highly non-uniform.